Though latest Linux-Zen is available in Arch Repo, it is not available in Manjaro as an installable option. Why not?
Kernels are not imported/used from Arch. Manjaro and maintains compiles its own kernels. And the linux-zen kernel isn’t an official one. It is probably from the AUR.
I think the AUR package maintainer hasn’t gotten 'round to providing it yet, that’s all.
I might be completely wrong in with my reasoning, in which case someone who does know can help me. I reserve the right to decide if you know or not though…
Install Arch if you really want these Kernel-packages.
Manjaro has its own kernels, which are handled by mhwd, and largely incorporate many of the liqorix and zen patches (where widely applicable).
So - mainly there is simply no need.
If you are specialized enough to know the differences, think you need them, and can install or compile them on your own (and keep the system running while working outside of mhwd) then you are welcome to do so. It is far from impossible.